I tried adding an AUX jack to a car stereo with an internal CD player and no native support for one.

When I first did tests, simply attaching to one of the amp’s input lines while the stereo was at min volume (1, not 0, more on this later) worked wonderfully.

To install the jack itself however, I thought that it may be dangerous to have the stereo send current to the AUX device and vice versa, so I thought about installing a switch to cut the stereo’s audio input lines when using the jack and vice versa.

I thought about just muting the stereo, or turning the volume to 0, or using a dummy paused CD, but all of the three activate the mute line on the amp, which isn’t a simple binary on/off deal, and activating it would also mute the jack.
Image

This is the original schematic of the amp circuit: Image
I attached to the IN1-4 lines like this: Image

I left the rear channels disconnected because a friend of mine suggested jumping two input lines could cause issues to the amp

However, once all was done, it wasn’t working correctly. First of all, when I flip the switch, some signal from the stereo still gets through. It’s very faint and you have to turn the volume way up to hear it, but it’s there. This concerns me if it may lead to damage to either the plugged in device or the stereo, and if it could be the cause of the following issue.

Second, when a male to male cable is plugged in the other end (without even anything attached to the other end), a high pitched whine starts playing from the speakers. I tried bypassing the jack with alligator clips to the rear terminals, but it still happens, and when this happens I suddenly read a resistance of 7~8MΩ between either of the jack’s channels and ground. I haven’t tried another cable, but if I test the cable itself when it’s unplugged I can’t read anything between the rings.

I’m especially puzzled as to why there were no issues when I tried by tapping into the lines, but now that I did it “”“properly”“” (at least in my head) it’s problematic.

The full schematic for the stereo can be found by googling “BLAUPUNKT FIAT B-MPV CD-MP3 7645324316 SM”